1. Field of the Invention
The present invention relates to an inner shield assembly in a color cathode ray tube, more particularly to an inner shield assembly which is capable of minimizing an external magnetism and a variation of magnetism at the time of rotation of direction of the color cathode ray tube so that degradation of color purity of a screen can be prevented by improving a structure of the inner shield assembly and by optimizing magnetic characteristic of a component material of the improved inner shield assembly.
2. Description of the Related Art
FIG. 1 shows a partial sectional view for illustrating a structure of a general color cathode ray tube.
Referring to FIG. 1, the color cathode ray tube, which is primary element for displaying pictures in picture display devices such as television receivers and computer monitors, is generally comprised of a panel 1 provided at the front and a funnel 2 provided at the rear.
In addition, the inside of the flat cathode ray tube, which is divided by the panel 1 and the funnel 2, includes a fluorescent screen 3 that served as a light emission, an electron gun (not shown) provided in the inside a neck of the funnel 2 for projecting electron beams 11 for light-emitting the fluorescent screen 3, a shadow mask 4 for selecting color of electron beams 11 from the electron gun (not shown), a frame assembly 5 including a main frame 5a for applying tension to the shadow mask 4 and a sub-frame 5b for supporting the main frame 5a, a spring 6 provided at a side of the main frame 5a for combining the frame assembly 5 and the panel 1, an inner shield 7 welded and fixed to the sub-frame 5b for shielding an external earth magnetism, and a reinforcement band 10 provided at the side of the panel 1 for preventing an external impact.
Further, the outside of the neck of the funnel 2 has a deflection yoke 8 for deflecting up-and-down and left-and-right the electron beams 11 projected from the electron gun (not shown) and two, four and six pole magnets 9 for correcting traveling tracks of the electron so that the projected electron beams 11 hit accurately on a fluorescent substance, thereby degradation of color purity is prevented.
On the other hand, as shown in FIGS. 2 and 3, when electron beams projected from the electron gun (not shown) in the cathode ray tube are affected by an external magnetism, traveling direction of electron beams is changed and the electron beams do not correctly hit on desired fluorescent substance 3a of the fluorescent screen 3 but hit on a black matrix 3b or other fluorescent substance. This event is referred to as xe2x80x9cmislandingxe2x80x9d. Accordingly, the inner shield 7 is used to shield affection by the external magnetism.
FIGS. 4a and 4b are a deployed view and an assembled view, respectively, for illustrating an example of an inner shield assembly structure of a conventional color cathode ray tube.
As shown in FIGS. 4a and 4b, the inner shield 7 comprises a pyramid-shaped main body 7a and a beam shield 7b combined to a forward end portion of the main body 7a. Also, the inner shield 7 is manufactured using material having a high permeability in order to minimize affection by an external magnetism.
FIGS. 5a and 5b are a deployed view and an assembled view, respectively, for illustrating another example of an inner shield assembly structure of a conventional color cathode ray tube.
As shown in FIGS. 5a and 5b, the inner shield 17 can be formed with an integration of a main body 17a with a beam shield 17b. 
However, in a magnetism shield structure such as the conventional inner shields 7 and 17, it is not bonded to the sub-frame 5b, although the shadow mask 4 is bonded up-and-down to the main frame 5a of the frame assembly 5 such that the shadow mask has a portion of the magnetism shield function. Accordingly, since a large space is presented between the shadow mask 4 and the sub-frame 5b, there is a problem that the magnetism shield function cannot be performed in this space.
In addition, since an external magnetism shield effect is affected by the magnetic characteristic of material as well as the inner shield assembly structure, there is a need to adjust the magnetic characteristic of component material of the inner shield assembly in order to maximize the magnetism shield function.
Accordingly, the present invention has been made the aforementioned problems of the prior art in mind, and an object of the present invention is to provide an inner shield assembly which is capable of maximizing an external magnetism shield effect by improving a structure of the inner shield assembly and optimizing magnetic characteristic of component material of the improved inner shield assembly.
In order to accomplish the above object, according to the present invention, an inner shield assembly provided at the rear side of a frame assembly having a main frame and a sub-frame for shielding external magnetism including a main body for shielding the inside of a funnel, a rectangular beam shield combined to the front of the main body, and a front portion formed extending from the edge to the front of the beam shield around the outside of the shadow mask and the frame assembly, wherein material of the front portion has maximum permeability of at least 3000 and coercivity of at most 1.25 Oe.
Preferably, the magnetic characteristic of each of material is adjusted to have an interrelation of 0.5xe2x89xa6xcexc2/xcexc1xe2x89xa61.5, where, maximum permeability of material of the main body is xcexc1 and maximum permeability of material of the front portion is xcexc2. In addition, the magnetic characteristic of each of material is adjusted to have an interrelation of 0.5xe2x89xa6H1/H2xe2x89xa63.0, where, coercivity of material of the main body is H1 and coercivity of material of the front portion is H2. As a result, an optimal magnetism shield effect can be obtained.